The field of this invention is subsurface safety valves and more particularly, sand seals for insert safety valves, generally installed on wireline.
Production tubing generally includes a subsurface safety valve (SSV) as part of the string. If the SSV malfunctions, an insert safety valve can be lowered through the tubing string, generally on a wireline, so that it seats in a pair of seal bores which permit the existing hydraulic control line system for the tubing SSV to be used in operation of the insert safety valve. The downhole safety valves previously used employed a shifting flow tube actuated by an annularly shaped piston using the hydraulic pressure in the control line. The piston would move against the opposing force of a return spring. Downward movement of the flow tube would rotate a flapper 90 degrees and away from a mating seat to allow flow to pass uphole through the flow tube. The piston resided in an annular recess in the housing formed between an inner wall and an outer wall. The flow tube was positioned inwardly of the inner wall leaving a clearance. The clearance was necessary to allow the flow tube to freely translate, as needed to open or close the SSV or the similarly operating insert safety valve.
In operation, applied pressures in the hydraulic control system had to exceed the operating tubing pressures to stroke the flow tube downwardly. In some instances, the applied control pressure was sufficient to flex the inner housing wall. Since the gap existed between the flow tube and the inner housing wall by design and well fluids could migrate into that gap, the flexing of the inner housing wall could cause seizure of the flow tube particularly when sand or grit was present in the well fluids. One solution that has been attempted is to enlarge the clearance between the flow tube and the housing inner wall. The disadvantage of this approach was that it would allow more sand and grit to reach sensitive areas such as the seals for the actuating piston. Accumulations in this sensitive seal area soon would cause a piston seal failure or seizure of the actuating piston. Another approach was to increase the wall thickness of the inner housing wall to minimize its deflection in response to applied control system pressures, which exceeded tubing pressure. However, this approach had the drawback of decreasing the flow tube bore size, which could impede production or limit the size of tools that could pass through the flow tube.
Another problem with insert safety valves when installed on wireline, particularly when it comes to large sizes such as 9⅝xe2x80x3 is the weight and length of the assembly. In the past, spacers and locks associated with an insert valve, particularly in the large sizes would constitute an assembly whose weight could exceed the capability of the wireline. Additionally, the length of large size assemblies could exceed the available length in a surface lubricator. This could necessitate the use of non-conventional lubricators, which added expense. The undue length could also be an issue in a deviated well where a potential of getting the insert valve assembly stuck existed.
The present invention has as one of its objectives the ability to effectively exclude or at least minimize the adverse effect of sand or grit in the clearance between the flow tube and the inner housing wall. This objective is accomplished without the disadvantages of the prior attempts described above. Another objective of the invention is to shorten the assembly length and weight so as to facilitate delivery of an insert valve with standard wireline equipment and lubricators. Those skilled in the art will appreciate how these objectives are met by a review of the description of the preferred embodiment, which appears below.
A seal is provided to prevent contamination by sand or grit in the clearance between a flow tube and the inner housing wall in a safety valve. An enlarged space is provided adjacent the seal to allow accumulation of sand or grit in the annular clearance space without causing seizure of the flow tube. The insert safety valve is assembled without a spacer with a seal mounted to the lower end of the insert valve to engage the bottom sub on the SSV. As a result, particularly for larger sizes of insert safety valves, the assembly is lighter and shorter, which facilitates use of readily available standard lubricator and wireline equipment.